Soot is a product of combustion in the combustion chamber of an engine, and it transfers in small amounts to crankcase oil in an engine, particularly a diesel engine. Thus, during operation of an engine, oil gradually builds up soot. When soot in the oil reaches an unacceptable level, the lubricating ability of the oil is diminished. Thus, measuring the soot content in oil over time is desirable as an indicator of the need to replace the oil.
Soot particles are very fine conductive particles. Despite this conductivity, they can increase the relative permittivity or relative dielectric constant of a dielectric fluid, such as oil, by acting as an artificial dielectric. Thus, one method of determining the soot content of oil involves measuring relative permittivity, such as taught in expired U.S. Pat. No. 4,345,202 to Nagy et al., the entire contents of which is incorporated herein by reference.
FIG. 5 shows an apparatus used in the method of Nagy et al. A Gunn oscillator 100 receives a supply voltage from a voltage supply line 102 coupled to one end of a sealed coaxial transmission line 104 having outer and inner conductors 104A and 104B, respectively. The other end of the transmission line 104 comprises a probe 106 having the same inner conductor 104B as line 104 and an outer conductor 108 made from a fine mesh screen connected to outer conductor 104A. The bottom of the probe 106 is shorted by a conductor such as a mesh screen 110, and the probe 106 is immersed in the engine oil. Microwave energy from the oscillator 100 is reflected back by the short 10 to produce a standing wave 112 within the transmission line 104. A diode detector is physically moved along a transmission line to find a voltage null in the standing wave, which null changes with the percentage of soot. In FIG. 5, a plurality of microwave diode detectors 114, 116 and 118 are longitudinally spaced along the transmission line 104 for illustrative purposes. When the location of the null voltage is found, it is compared to the location of the null voltage point with no soot in the oil. From this comparison, the level of soot in the oil is determined.
While the teachings of Nagy et al. were applied easily in laboratory settings, their implementation in an actual automotive sensor proved difficult.